The present disclosure relates to a light emitting element, an illumination device, and a display apparatus, and more particularly to a light emitting element using electroluminescence of an organic material, an illumination device and a display apparatus using the light emitting element.
A light emitting element (hereinafter, referred to as an “organic EL element”) using electroluminescence of an organic material has attracted attention as a light emitting element which can emit light with high luminance through low voltage DC driving and has been actively researched and developed. The organic EL element generally has a structure where an organic layer including an emission layer having a thickness of about several tens to several hundreds of nm is interposed between a reflective electrode and a transmissive electrode. In such an organic EL element, light beams emitted from the emission layer interfere with each other inside the element structure, and are extracted to outside. In the related art, an attempt for improving emission efficiency of the organic EL element using the interference has been made.
Japanese Unexamined Patent Application Publication No. 2002-289358 has proposed a method in which the emission efficiency is increased by setting a distance between the emission position and the reflective layer such that light of an emission wavelength becomes resonant by the use of the interference between light generated in the direction of the transmissive electrode direction from the emission layer and light generated in the direction of the reflective electrode.
Japanese Unexamined Patent Application Publication No. 2000-243573 prescribes both the distance from the emission position to the reflective electrode and the distance from the emission position to the interface between the transmissive electrode and the substrate, in consideration of light reflection from the interface between the transmissive electrode and the substrate.
In Pamphlet of International Publication WO. 01/039554, the emission efficiency is increased by setting the thickness of a layer between the transmissive electrode and the reflective electrode such that light of a desired wavelength becomes resonant by the use of interference occurring due to multiple reflections of light between the transmissive electrode and the reflective electrode.
The specification of Japanese Patent No. 3508741 has proposed a method in which an attenuation balance of three colors, red (R), green (G), and blue (B) is controlled by controlling the thickness of the organic layer, as a method of improving a viewing angle characteristic of the white chromaticity points in a display apparatus where light emitting elements are combined so as to increase the emission efficiency using a resonator structure.
However, in the techniques in the related art, in the organic EL element using the interference of the emitted light in order to increase the emission efficiency, if the bandwidth of the interference filter of the extracted light h is reduced, a wavelength of the light h is large and shifted when the emission surface is viewed from the tilt direction, thereby reducing emission intensity, which leads to an increase in viewing angle dependency of the emission characteristics.
In contrast, Japanese Unexamined Patent Application Publication No. 2006-244713 has proposed a method in which an emission phase by the reflective layer is set to have a reverse phase to a central wavelength in the perfect reflective surface side and the exit side in the organic EL element having a narrow monochromatic spectrum of a half value width, thereby suppressing color variations due to the viewing angle.
In addition, since the interference occurs inside of the element even in the organic light emitting element having white emission layers which are sequentially laminated, in order to efficiently extract white emission having a wide wavelength component, the emission position is close to the reflective layer, particularly, is close at a distance of 80 nm or less. If the emission position is distant from the reflective electrode layer and thus the distance therebetween is increased, it is difficult to obtain white emission having a wide spectrum.
Japanese Unexamined Patent Application Publication No. 2004-79421 discloses a method in which the distance from the emission position to the reflective layer and the distance from the emission position to the interface between the transmissive electrode and an external layer are prescribed, and thereby it is possible to obtain a light emitting element which has good efficiency and white chromaticity.
Japanese Unexamined Patent Application Publication No. 2006-244712 has reported that favorable white chromaticity points can be obtained by incorporating reverse-phase interference in the same manner as Japanese Unexamined Patent Application Publication No. 2006-244713. However, since phase cancellation is not performed in a wide wavelength band, suppression of variations in a color such as a single color due to the viewing angle is not disclosed in the same manner as Japanese Unexamined Patent Application Publication No. 2006-244713.
On the other hand, a technique has been used in which, in order to increase emission efficiency and improve emission life, the organic layer is configured to have a laminate (a so-called tandem structure) where the emission layers are connected in series to each other by laminating a plurality of emission layers via an intermediate layer (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-173550, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-511100, and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-518400). In this kind of emission layer, it is possible to laminate any number of emission layers. In addition to an increase in efficiency by laminating layers generating spectra of the same wavelength, it is possible to adjust colors by laminating layers generating spectra of the different wavelengths. For example, by laminating a blue emission layer generating blue and laminating a yellow emission layer generating yellow, white light can be generated as composite light thereof.
However, in a case where the above-described tandem structure is formed, it is difficult to form all the distances from the respective emission positions to the reflective layer so as to be equal to or less than 80 nm, and since the viewing angle dependency of luminance and colors is much increased, light distribution characteristics as an illumination device or display characteristics as a display apparatus are considerably deteriorated.